1. Field of the Invention PA1 2. Description of the Prior Art
The present invention relates to a gas friction pump including a cylindrical rotor element, a cylindrical stator element surrounding the rotor element, and a plurality of parallel, arranged beneath each other, discharge channels formed by spiral grooves and separated by webs, with the discharge channels forming a discharge chamber which provides for pumping gas from the pump suction port to the pump discharge port.
Different types of gas friction pumps are used for gas delivery. Their operation is based on the transfer of pulses from movable walls to gas particles. In this way, a gas flow in a predetermined direction is created. Gas friction pumps, which function in a pressure region in which a free path length of gas molecules is large in comparison with geometrical dimensions of a pump, i.e., which function in a molecular flow region, are called molecular pumps.
A first gas friction pump of this type was proposed by Gaede. A further modification of a Gaede pump, without changing its basic principle, was made by Siegbahn. In Siegbahn pump, a rotatable disc is used as a movable wall.
Another modification of the Gaede pump was made by Holweck. In the Holweck pump, a cylinder surface serves as a movable wall.
A large advance in a further development of gas friction pump was made by Becker. In the pump construction of Becker, movable and stationary walls are alternatively arranged one behind the other, with both movable and stationary walls being formed as turbine disc provided with vanes. These pumps were called turbomolecular pumps.
All of the above-mentioned gas friction pumps play an ever increasing role in vacuum technology, in particular, in high and ultrahigh vacuum technology. At that, the Becker turbomolecular pump is used on one side of a vacuum system, and a gas friction pump of Gaede, Holweck, or Siegbahn is used on the opposite side. A multi-stage turbomolecular pump permits to obtain high pressure ratios and, therefore, is particularly suitable for use in a high and ultrahigh vacuum region. However, their application range is limited by their inability to operate in the region of higher pressures. Therefore, because of large distances between the pump elements they are fully operational only at low pressures of about 10.sup.-3 mbar.
The Gaede, Siegbahn and Holweck gas friction pumps are suitable for application in the above-discussed pressure region. They can be used in this region separately or be consecutively connected with a turbomolecular pump. The combination of turbomolecular pumps with gas friction pumps permits to shift the operational region of the turbomolecular pumps toward the region of higher discharge pressures.
However, the gas friction pumps have certain drawbacks which adversely affects their operation. It is important for a proper operation of the gas friction pumps that the distance between rotatable and stationary elements be very small to keep the backstreaming and discharge losses to a minimum. This is particularly applicable to Gaede, Siegbahn and Holweck pumps. In addition, these pumps, as well as the turbomolecular pumps, can function in the high pressure region and molecular flow region only then when the distance between the rotatable and stationary elements is small in comparison with the mean free path length of the molecules of a pumped gas. Only then, the gas friction pumps can achieve the full pressure ratio in the molecular flow region.
A narrow rotor-stator split is a necessary premise for proper functioning of the gas friction pumps. However, a narrow split leads to small dimensions of the discharge chamber and, thus, results in a limited suction capacity. Therefore, the gas compressed in a turbomolecular pump can be further upgraded only to a definite magnitude, so that its suction capacity is limited toward higher pressures. In order to further expand the operational range of the turbomolecular pumps toward a higher pressure region, they should be combined with gas friction pumps with a high suction capacity the geometrical dimensions of which permits them to operate in a molecular flow region.
The Gaede and Siegbahn gas friction pumps, because of their construction, cannot be modified so that their suction capacity substantially increases, without an adverse affect on their basic function. Moreover, they have specific drawbacks which reduce their efficiency in certain applications. For example, in the Siegbahn gas friction pump, the gas is pumped against a centrifugal force.
Accordingly, an object of the present invention is a gas friction pump operable in the molecular flow range and having a higher suction capacity than the conventional gas friction pumps.
Another object of the present invention is a gas friction pump the geometrical dimensions of which are comparable with the geometrical dimensions of conventional gas friction pumps.
A further object of the present invention is a gas friction pump operable in a combination with a turbomolecular pump.